1. Statement of the Technical Field
The inventive arrangements relate generally to RF devices and more particularly to structures and systems for preventing degradation of fluid dielectrics that are used in RF devices.
2. Description of the Related Art
Glass ceramic substrates calcined at 850° C. to 1,000° C. are commonly referred to as low-temperature co-fired ceramics (LTCC). This class of materials have a number of advantages that make them especially useful as substrates for RF systems. For example, low temperature 951 co-fire Green Tape™ from Dupont® is Au and Ag compatible, and it has a thermal coefficient of expansion (TCE) and relative strength that are suitable for many applications. The material is available in thicknesses ranging from 114 μm to 254 μm and is designed for use as an insulating layer in hybrid circuits, multi-chip modules, single chip packages, and ceramic printed wire boards, including RF circuit boards. Similar products are available from other manufacturers.
LTCC substrate systems commonly combine many thin layers of ceramic and conductors. The individual layers are typically formed from a ceramic/glass frit that can be held together with a binder and formed into a sheet. The sheet is usually delivered in a roll in an unfired or “green” state. Hence, the common reference to such material as “green tape”. Conductors can be screened onto the layers of tape to form RF circuit elements antenna elements and transmission lines. Two or more layers of the same type of tape is then fired in an oven. The firing process shrinks all of the dimensions of the raw part. Accordingly, it is highly important that the material layers all shrink in a precise, predetermined way that will provide consistent results from one module to the next.
Recent interest in fluid dielectric materials suggest the use of LTCC as a substrate because of its known resistance to chemical attack from a wide range of fluids. The material also has superior properties of wetability and absorption as compared to other types of solid dielectric material. These factors, plus LTCC's proven suitability for manufacturing miniaturized RF circuits, make it a natural choice for use in RF devices incorporating fluid dielectrics.
Still, the use of fluid dielectrics raises new potential problems. For example, fluid dielectrics can suffer degradation from a variety of factors. For example, the degradation can occur due to temperature variations, micro-gravity, phase separation, particulate settling and orientation, ionic migration, dendrite growth, and other intrinsic molecular separation phenomena. Some of these problems are less likely to occur in dynamic systems. However, even in the case of dynamic systems, fluids can separate due to particle fallout, particle separation, sedimentation, eddy effects and so on. These kinds of fluid degradations will effect the overall electrical characteristics of the fluid dielectric, regardless of whether the fluid is a dielectric suspension, dielectric agglomerate, a dielectrically loaded fluid, or a polymer blend.
In order to overcome the foregoing limitations, RF systems that take advantage of fluid dielectrics should include agitation systems for proper maintenance of the fluid dielectric. However, conventional agitations systems can cause damage to the fluid dielectric by introducing shear stresses. They can also have the undesired effect of consuming the limited space available on circuit boards. More advanced systems can potentially avoid some of the problems associated with shear stresses but can in some instances produce less effective mixing.